Multi-mutant diphtheria toxin vaccines

ABSTRACT

Disclosed is DNA encoding diphtheria toxin polypeptides having multiple mutations, which render the polypeptides useful as vaccines.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 09/250,131, Feb. 16, 1999 now U.S. Pat. No. 6,455,673, which is a continuation-in-part of U.S. Ser. No. 08/257,781, filed Jun. 8, 1994, now U.S. Pat. No. 5,917,017.

This invention was made at least in part with funding from the U.S. government (NIH grant AI 22021). The government has certain rights in this invention.

FIELD OF THE INVENTION

This invention relates to vaccines that protect against diphtheria toxin.

BACKGROUND OF THE INVENTION

Diphtheria toxin (DT) is a protein exotoxin produced by the bacterium Corynebacteria diphtheria. The DT molecule is produced as a single polypeptide that is readily spliced to form two subunits linked by a disulfide bond, Fragment A (N-terminal˜f21K) and Fragment B (C-terminal ˜37K), as a result of cleavage at residue 190, 192, or 193 (Moskaug, et al., Biol Chem 264:15709–15713, 1989; Collier et al., Biol Chem, 246:1496–1503, 1971). Fragment A is the catalytically active portion of DT. It is an NAD-dependent ADP-ribosyltransferase which specifically targets a protein synthesis factor termed elongation factor 2 (EF-2), thereby inactivating EF-2 and shutting down protein synthesis in the cell. Fragment A consists of the diphtheria toxin C domain. Fragment A is linked to the diphtheria toxin Fragment B by a polypeptide loop. Fragment B of DT possesses a receptor-binding domain (the R domain) which recognizes and binds the toxin molecule to a particular receptor structure found on the surfaces of many types of mammalian cells. Once DT is bound to the cell via this receptor structure, the receptor/DT complex is taken up by the cell via receptor-mediated endocytosis. A second functional region on Fragment B (the T domain) acts to translocate DT across the membrane of the endocytic vesicle, releasing catalytically active Fragment A into the cytosol of the cell. A single molecule of Fragment A is sufficient to inactivate the protein synthesis machinery in a given cell.

Immunity to a bacterial toxin such as DT may be acquired naturally during the course of an infection, or artificially by injection of a detoxified form of the toxin (i.e., a toxoid) (Germanier, ed., Bacterial Vaccines, Academic Press, Orlando, Fla., 1984). Toxoids have traditionally been prepared by chemical modification of native toxins (e.g., with formalin or formaldehyde (Lingood et al., Brit. J. Exp. Path. 44:177, 1963)), rendering them nontoxic while retaining an antigenicity that protects the vaccinated animal against subsequent challenges by the natural toxin: an example of a chemically-inactivated DT is that described by Michel and Dirkx (Biochem. Biophys. Acta 491:286–295, 1977), in which Trp-153 of Fragment A is the modified residue.

Another method for producing toxoids is by the use of genetic techniques. Collier et al., U.S. Pat. No. 4,709,017 (herein incorporated by reference) disclosed a genetically engineered diphtheria toxin mutant that bears a deletion mutation at Glu-148 of diphtheria toxin. Glu-148 was originally identified as an active-site residue by photoaffinity labelling (Carroll et al., Proc. Natl. Acad. Sci. USA 81:3307, 1984; Carroll et al. Proc. Natl. Acad. Sci. USA 82:7237, 1985; Carroll et al., J. Biol. Chem. 262:8707, 1987). Substitution of Asp, Gln or Ser at this site diminishes enzymatic and cytotoxic activities by 2–3 orders of magnitude, showing that the spatial location and chemical nature of the Glu-148 side-chain greatly affects these activities (Carroll et al., J. Biol. Chem. 262:8707, 1987; Tweten et al., J. Biol. Chem. 260:10392, 1985; Douglas et al., J. Bacteriol. 169:4967, 1987). Similarly, Greenfield et al., U.S. Pat. No. 4,950,740 (herein incorporated by reference) disclosed genetically engineered mutant forms of DT in which the Glu-148 residue is deleted or replaced with Asn, and the Ala-158 residue is replaced with Gly. The DNA sequence and corresponding amino acid sequence of wild-type diphtheria toxin DNA are set forth in FIG. 1 (SEQ ID NOs:1 and 2, respectively).

SUMMARY OF THE INVENTION

The invention features diphtheria toxoids having multiple mutations as compared with wild-type diphtheria toxin. Thus, the invention features a polypeptide having a mutant diphtheria toxin C domain, a mutant diphtheria toxin T domain, and a mutant diphtheria toxin R domain, wherein the C domain has a mutation at Glu148, the T domain has a mutation at Glu349, and the R domain has a mutation at Lys 516 and/or Phe530 of wild-type diphtheria toxin. In various embodiments, the polypeptide includes any or all of the following mutations: Glu148Ser, Glu349Lys, Lys516Ala, and/or Phe530Ala.

The invention also features a polypeptide having a mutant diphtheria toxin C domain, a mutant T domain, and a mutant loop connecting the diphtheria toxin C and T domains, wherein the C domain has a mutation at Glu148, the T domain has a mutation at Glu349, and the loop has a mutation at Arg190, Arg192, and/or Arg193 of wild-type diphtheria toxin. In various embodiments, the polypeptide (or a mixture of polypeptides) includes any or all of the following mutations: Glu148Ser, Glu349Lys, Arg190Ser, Arg192Gly and/or Arg193Ser. In addition, all of the polypeptides of the invention bind sensitive cells with less affinity than does wild-type diphtheria toxin, and are capable of forming an immune complex with an antibody that specifically recognizes the R domain of wild-type diphtheria toxin.

These so-called “multi-mutant” diphtheria toxoids of the invention can be used as vaccines to provide immunoprotection against diphtheria toxin and against infection by Corynebacteria diphtheriae. One approach to vaccination utilizes live, genetically engineered microorganisms (cells or viruses) expressing mutant toxin genes. The multi-mutant toxoids of the invention, and the DNAs which encode them, carry significantly less risk of reversion than do single residue deletion mutants, and so are good candidates for use in a live, genetically engineered vaccine cell that is capable of proliferating in the vaccinee. As discussed below, acellular vaccines also are within the invention.

The invention also includes vectors (e.g., plasmids, phages and viruses) including DNA sequences encoding the diphtheria toxoid mutants described herein. Expression of a diphtheria toxoid polypeptide of the invention can be under the control of a heterologous promoter, and/or the expressed amino acids can be linked to a signal sequence. A “heterologous promoter” is a promoter region that is not identical to the promoter region found in a naturally occurring diphtheria toxin gene. The promoter region is a segment of DNA 5′ to the transcription start site of a gene, to which RNA polymerase binds before initiating transcription of the gene. Nucleic acids encoding a diphtheria toxoid of the invention can be prepared as an essentially pure preparation, which is a preparation that is substantially free of other nucleic acid molecules with which a nucleic acid encoding diphtheria toxin is naturally associated in Corynebacterium. A nucleic acid encoding a diphtheria toxoid of the invention can be contained in a cell, or a homogeneous population of cells, preferably a B. subtilis, Bacillus Calmette-Guerin(BCG), Salmonella sp., Vibrio cholerae, Corynebacterium diphtheriae, Listeriae, Yersiniae, Streptococci, or E. coli cell. The cell is capable of expressing the diphtheria toxoid polypeptide of the invention.

Diphtheria toxoids that are “immunologically cross-reactive” possess at least one antigenic determinant in common with naturally occurring diphtheria toxin, so that they are each bound by at least one antibody with specificity for naturally occurring diphtheria toxin. A diphtheria toxoid of the invention is immunologically cross-reactive with naturally occurring diphtheria toxin and possesses at least one of the mutations described herein.

The invention includes various vaccines that can be used to immunize a mammal (e.g., a human) against progression of the disease diphtheria, and against infection by the bacterium Corynebacterium diphtheriae. A vaccine of the invention can include any of the various DNAs encoding a diphtheria toxoid of the invention. Alternatively, a cell or virus expressing a nucleic acid of the invention, e.g., a live vaccine cell, can be used as a vaccine. Examples of suitable cells include B. subtilis, BCG, Salmonella sp., Vibrio cholerae, Listeriae, Yersiniae, Streptococci, Corynebacterium diphtheriae, and E. coli. A “live vaccine cell” can be a naturally avirulent live microorganism, or a live microorganism with low or attenuated virulence, that expresses an immunogen. A killed-cell vaccine can also be used.

One method for manufacturing a vaccine of the invention includes culturing a cell containing DNA encoding a diphtheria toxoid of the invention under conditions permitting proliferation of the cell and expression of the DNA, the cell being one that is suitable for introduction into an animal as a live-cell vaccine. The vaccine can be used in a method of immunizing a mammal against diphtheria by introducing an immunizing amount of a vaccine of the invention into the mammal.

In an alternative method of vaccination, an acellular vaccine that includes a nucleic acid encoding a diphtheria toxoid of the invention is introduced into the mammal. For example, a DNA vaccine can be administered by biolistic transfer, a method of delivery involving coating a microprojectile with DNA encoding an immunogen of interest, and injecting the coated microprojectile directly into cells of the recipient (Tang et al., Nature 356:152–154, 1992; hereby incorporated by reference). The diphtheria toxoid of the invention is then expressed from the DNA to stimulate an immune response in the recipient.

The polypeptides can be made by any of a variety of conventional methods, such as by culturing any of the various cells containing a DNA encoding a diphtheria toxoid of the invention under conditions permitting expression of the DNA. Included in the invention is an isolated mutant diphtheria toxin polypeptide, an “isolated” polypeptide being one that is substantially free of cellular material, viral material, culture medium (when produced by recombinant DNA techniques), or chemical precursors or other chemicals (when chemically synthesized). Generally, the polypeptide is a substantially pure preparation, meaning that at least 50% (by weight) (e.g., at least 75%, 90%, or 99%) of the protein present in the preparation is the diphtheria toxoid polypeptide of the invention.

A vaccine against diphtheria toxin can be formulated as a composition that includes a diphtheria toxoid polypeptide of the invention and an adjuvant. Examples of adjuvants include, but are not limited to, aluminum salts, bacterial endotoxins, Bacillus Calmette-Guerin (BCG), liposomes, microspheres (i.e., microencapsulation polymers used in orally administered vaccines), and Freund's complete or incomplete adjuvant. An “adjuvant” is a substance that increases the immunogenicity of an antigen.

If desired, the diphtheria toxoid polypeptides of the invention can be covalently attached to a moiety, e.g., a polysaccharide or a second polypeptide. The moiety can serve as a carrier substance for the polypeptide or, alternatively, the diphtheria toxoid polypeptide of the invention can serve as a carrier substance for the moiety, preferably enhancing the immunogenicity of the moiety. A “carrier substance” is a substance that confers stability on, aids, and/or enhances the transport or immunogenicity of an associated molecule.

A diphtheria toxoid of the invention can also be prepared as a fusion polypeptide that includes a diphtheria toxoid polypeptide covalently linked to a second polypeptide. The fusion polypeptide can be formulated as a vaccine, which can be used to immunize a mammal (e.g., a human patient) against diphtheria toxin or infection by Corynebacterium diphtheriae. The fusion polypeptide can be administered directly to the mammal in a method of immunization, or it can first be combined with an adjuvant. Alternatively, the DNA encoding the fusion polypeptide can be used directly as a vaccine, or it can be incorporated into a cell (e.g., a live vaccine cell) capable of expressing the fusion polypeptide, which cell can be used as a vaccine against diphtheria toxin. A “fusion polypeptide” is a polypeptide in which a diphtheria toxoid of the invention is linked to a second polypeptide sequence, typically by expression of a genetically engineered hybrid DNA.

The mutant diphtheria toxoids of the invention can be safely administered to a mammal in the form of an acellular polypeptide, a live attenuated vaccine strain that expresses the toxoid or a nucleic acid that expresses the toxoid in the vaccinee. The diphtheria toxoids of the invention are enzymatically dysfunctional and substantially free of any risk of reversion, even in a continuously proliferating microbial host.

Other features and advantages of the invention will be apparent from the following detailed description and from the claims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a representation of the nucleotide sequence and corresponding amino acid sequence of wild-type diphtheria toxin (SEQ ID NOS:1 and 2, respectively).

DETAILED DESCRIPTION

Preparation and Analysis of Mutant Diphtheria Toxoids

Mutant diphtheria toxoids can be generated by oligonucleotide-directed mutagenesis of the diphtheria toxin gene, as described below. The mutant genes can then be expressed in E. coli or any other standard expression system by conventional methods, and, if desired, extracts can be assayed for NAD:EF-2 ADP-ribosyltransferase activity and diphtheria toxin-specific protein by Western blot analysis using standard methods.

EXAMPLES

Two preparations of mutant diphtheria toxin polypeptides were prepared using the following procedure. The polypeptides in these preparations were mutated as follows: (i) Glu148Ser, Glu349Lys, Lys516Ala, and Phe530Ala; and (ii) Glu148Ser, Arg190Ser, Arg192Gly, Arg193Ser, and Glu349Lys. To produce the mutants, a gene encoding a diphtheria toxin polypeptide having the mutation Glu148Ser was further mutated to create the multi-mutant polypeptides (see, e.g., Fu et al., “Selection of Diphtheria Toxin Active-Site Mutants in Yeast” in ADP-Ribosylation in Animal Tissues, ed. Haag and Koch-Nolte, Plenum Press, NY). This mutant diphtheria toxin was engineered to have a hexa-histidine tag at its carboxyl terminus in order to facilitate purification of the polypeptide by affinity chromatography. The mutant diphtheria toxin gene was cloned into the BamHI and XhoI sites of pET22b, an E. coli expression vector. To clone the diphtheria gene into the BamHI and XhoI sites of pET22b, the synthetic oligonucleotides 5′GCC GCG GAT CCG GGC CTG GAT GAT GTT G3′ (SEQ ID NO:3) and 5′CGC CCG CTC GAG GCT TTT GAT TTC AAA3′ (SEQ ID NO:4) respectively, were used.

Using the mutant diphtheria toxin gene as a template, additional mutations were introduced into the gene by site-directed mutagenesis as follows. To substitute Glu for Ser at position 148 of cloned diphtheria toxin gene, the mutagenic oligonucleotides 5′ GGG AGT TCT AGC GTT AGC TAT ATT AAT AAC TGG3′ (SEQ ID NO:5) and 5′CCA GTT ATT AAT ATA GCT AAC GCT AGA ACT CCC3′ (SEQ ID NO:6) were used. To substitute Arg for Ser, Arg for Gly and Arg for Ser at position 190, 192 and 193 of cloned diphtheria toxin gene, the mutagenic oligonucleotides 5′GCA GGA AAT TCG GTC GGC TCG TCA GTA GGT AGC3′ (SEQ ID NO:7) and 5′GCT ACC TAC TGA CGA GCC GAC CGA ATT TCC TGC3′ (SEQ ID NO:8) were used. To substitute Glu for Lys at position 349, the mutagenic primers 5′CCA TTG GTA GGA AAA CTA GTT GAT ATT GGT3′ (SEQ ID NO:9) and 5′ ACC AAT ATC AAC TAG TTT TCC TAC CAA TGG3′ (SEQ ID NO:10) were used. To substitute Lys for Ala at position 516, the mutagenic primers 5′GGG TAC CAG GCA ACA GTA GAT CAC3′ (SEQ ID NO:11) and 5′GTG ATC TAC TGT TGC CTG GTA CCC3′ (SEQ ID NO:12) were used. To substitute Phe for Ala at position 530, the mutagenic primers 5′G CTA TCG CTA GCT TTT GAA ATC3′ (SEQ ID NO:13) and 5′ GAT TTC AAA AGC TAG CGA TAG C3′ (SEQ ID NO:14) were used. The amplified sequences were ligated together and transformed into the E. coli strain XL1-Blue. Plasmid DNA was amplified, purified, and sequenced to confirm the presence of the mutation(s). This procedure was repeated to introduce each mutation into the multi-mutant diphtheria toxins.

Plasmid DNA containing multiple mutations in the cloned diphtheria toxin gene was transformed into BL21 cells, an E. coli host cell for expression of the diphtheria toxin. Mutant diphtheria toxins were purified from periplasmic extracts as follows. Cultures were grown in a 5L fermentor in Luria broth containing ampicillin at 37° C. to an OD₆₀₀ of 0.8–1.0, and protein expression was induced by addition of isopropyl β-D-thiogalactopyroanoside (1 mM) for 3 hours at 28° C. Periplasmic proteins were extracted by first resuspending pelleted cells in 0.4 fermentation culture volumes of the following solution: 20% sucrose/1 mM EDTA/30 mM Tris-HCl (pH 8.0). After incubation at room temperature for 10 minutes, the mixture was centrifuged and the cell pellets were resuspended in the same volume of ice-cold 5 mM MgSO₄. After incubation on ice for 10 minutes, the mixture was centrifuged, and the resulting supernatant containing the desired protein was purified by affinity chromatography on a Ni⁺⁺ chelate column. The protein was then purified by gel filtration (on a Superdex 200 column) in a buffer of 20 mM Tris-HCl (pH 8.0) containing 150 mM NaCl. Proteins were purified to 95% homogeneity, as judged by SDS-PAGE. Approximately 3 mg of purified protein was obtained from one liter of fermentor culture. Proteins were stored in 20 mM Tris buffer (pH 8.0) at −20° C. until use.

Toxicity Assay

Standard methods of assaying the toxicity of diphtheria toxin mutants employ a diphtheria toxin-sensitive tissue culture cell line, which is a line of cells bearing the diphtheria toxin receptor, e.g., Vero or BSC1 cells. The cells are treated with a known amount of the mutant diphtheria toxin, with naturally occurring diphtheria toxin (as a positive control), or with a carrier protein such as bovine serum albumin (as a negative control). After incubation, viable colonies are counted to assess the extent of cell killing (see, e.g., Yamaizumi, M. et al. Cell 15:245–250, 1978). Alternatively, the extent of cell-killing can be determined by measuring the extent of inhibition of protein synthesis. After incubation with one of the diphtheria toxin samples described above, a radiolabelled amino acid (e.g., [¹⁴CC]Leu) is added to the growth medium of the cell culture, and the decline in de novo protein synthesis is measured, e.g., by scintillation counting of TCA-precipitable protein. Such methods are routine and known to one skilled in the art. The mutants described above were assayed for residual toxicity using toxin-sensitive Vero cells. The mutant polypeptides subsequently were assayed for residual toxicity by intradermal inoculation of up to 50 μg of toxoid into each guinea pig using conventional methods. The animals were observed daily for changes in activity, appetite, behavior, and appearance. The toxicity results are set forth in Table 1. Both in Vero cell cultures and following intradermal inoculation, toxoids having the mutations (i) Glu148Ser, Lys516Ala, Phe530Ala, and Glu349Lys and (ii) Arg190Ser, Arg192Gly, Arg193Ser, and Glu349Lys showed a significant reduction in toxicity, relative to the native diphtheria toxin.

Immunogenicity

After confirming that toxicity of the mutant toxoids of the invention is significantly reduced, the mutant diphtheria toxins were treated with formalin, and both formalinized and untreated diphtheria toxin samples were analyzed for immunogenicity as follows:

Guinea pigs and mice (another species that is naturally sensitive to the cell-killing effects of diphtheria toxin may be substituted) were immunized with each recombinant toxoid of the invention, separately, according to the following protocol: 1–50 μg recombinant toxoid, suspended in 250 μl of Alhydrogel (aluminum hydroxide) adjuvant, was subcutaneously injected into each of 8 guinea pigs per group. Ten groups of guinea pigs were used for each experiment over the course of one year, with 3 experiments performed per year. A 50 μl sample of the recombinant toxoid was injected into each mice. For mice, 10 animals were used per group, with 25 groups of mice used per experiment and 6 experiments performed over the course of one year. The guinea pigs and mice received two injections of the recombinant toxoid and were bled under anesthesia 2–6 times over the course of a year.

Blood samples were assayed for antitoxin antibodies by testing serial dilutions for reactivity to naturally occurring diphtheria toxin. Those animals that received high enough doses of toxoid to induce anti-toxoid formation can be challenged with wild-type diphtheria toxin, in order to determine whether the antibodies are immunoprotective. If desired, toxoids that induce a positive response in the above assay can be incorporated into live vaccines to protect against diphtheria toxin. The results of such immunogenicity assays are provided in Table 1. Appropriate live vaccine microorganisms (cells or viruses) genetically engineered to express a toxoid of the invention can be tested by injecting the vaccine into a DT sensitive animal, and, after a 2–3 month incubation period, challenging the animal with either a) a lethal dose of naturally occurring DT, or b) multiple, serially administered doses of naturally occurring DT, so as to calibrate the range of acquired immunity.

Preparation and Use of a Nucleic Acid Encoding a Diphtheria Toxoid

A DNA sequence encoding a diphtheria toxoid of the invention can be expressed by standard methods in a prokaryotic host cell. DNA encoding a diphtheria toxoid of the invention is carried on a vector and operably linked to control signals capable of effecting gene expression in the prokaryotic host. If desired, the coding sequence can contain, at its 5′ end, a sequence encoding any of the known signal sequences capable of effecting secretion of the expressed protein into the periplasmic space of the host cell, thereby facilitating recovery of the protein. By way of example, a vector expressing the diphtheria toxoid of the invention, or a fusion protein including the polypeptide of the invention, can include (i) an origin of replication functional in E. coli derived from the plasmid pBR322; (ii) a selectable tetracycline resistance gene also derived from pBR322; (iii) a transcription termination region, e.g., the termination of the E. coli trp operon (placed at the end of the tetracycline resistance gene to prevent transcriptional read-through into the trp promoter region); (iv) a transcription promoter, e.g., the trp operon promoter, or the diphtheria toxin promoter; (v) the protein coding sequence of the invention; and (vi) a transcription terminator, e.g., the T1T2 sequence from the ribosomal RNA (rrnB) locus of E. coli. The sequences of carrier molecules, the methods used in the synthesis of the DNA sequences, the construction of fusion genes, and the appropriate vectors and expression systems are all well known to those skilled in the art.

Similar expression systems can be designed for fusion polypeptides. For example, a nucleic acid sequence encoding the mutant diphtheria toxin can be fused to a sequence encoding a tag that facilitates purification of the fusion protein. For example, conventional recombinant DNA technology can be used to encode a hexa-histidine tag at the carboxyl terminus of the fusion protein. The hexa-histidine tag can subsequently facilitate purification of the fusion protein. These procedures are an example of, but are not limiting on, the methods of the invention.

A variety of prokaryotes, including various strains of E. coli can be used; however, other microbial strains can also be used, e.g., C. diphtheriae. Typical plasmid vectors contain replication origins, selectable markers, and control sequences derived from a species compatible with the microbial host. Commonly used prokaryotic expression control sequences (also referred to as “regulatory elements”) include promoters for transcription initiation, optionally with an operator, along with ribosome binding site sequences. Promoters commonly used to direct protein expression include the beta-lactamase (penicillinase), the lactose (lac) (Chang et al., Nature 198:1056, 1977) and the tryptophan (trp) promoter systems (Goeddel et al., Nucl. Acids Res. 8:4057, 1980) as well as the lambda-derived P_(L) promoter and N-gene ribosome binding site (Shimatake et al., Nature 292:128, 1981). Examples of microbial strains, vectors, and associated regulatory sequences are listed herein to illustrate, but not to limit, the invention.

Preparation and Use of a Polypeptide Vaccine

The mutant diphtheria toxoids of the invention can be expressed in any known protein expression system and then purified by standard means. Alternatively, diphtheria toxoids of the invention can be synthesized by organic chemical synthesis or produced as a biosynthesized polypeptide. Organic chemical synthesis can be performed by conventional methods of automated peptide synthesis, or by classical organic chemical techniques. The diphtheria toxoid polypeptides of the invention can be purified using conventional methods of protein isolation, e.g., methods including but not limited to precipitation, chromatography, immunoadsorption, or affinity techniques. The polypeptides can be purified from a microbial strain genetically engineered to express the diphtheria toxoid of the invention, or from a medium containing such a microbial strain. Typically, the purified diphtheria toxoid then is treated with formalin or formaldehyde to stabilize the protein, according to conventional methods.

The purified polypeptide may be combined with a suitable carrier (such as physiological saline); with an adjuvant that increases the immunogenicity of the toxoid (such as aluminum salts, bacterial endotoxins or attenuated bacterial strains (e.g., BCG or Bordetella pertussis), attenuated viruses, liposomes, microspheres, or Freund's complete or incomplete adjuvant)); and/or with additional toxoids or killed or attenuated vaccine organisms (to form a multivalent vaccine). Such a vaccine may then be administered to a human or non-human mammal by any acceptable method, including but not limited to oral, parenteral, transdermal and transmucosal delivery methods. Administration can be in a sustained release formulation using a biodegradable biocompatible polymer, such as a microsphere; by on-site delivery using micelles, gels or liposomes; or by transgenic modes (e.g., by biolistic administration of the DNA of the invention directly into the mammal's cells, as described by Tang et al., Nature 356:152–154, 1992, herein incorporated by reference). Generally, the polypeptide vaccine is administered in a dosage of 1–1,000 μg/kg body weight of the animal. Suitable dosages can readily be determined by one of ordinary skill in the art of medicine.

Preparation and Use of Live Recombinant Vaccines

Appropriate live carrier organisms include attenuated microorganisms such as BCG, Salmonella sp., Vibrio cholerae, Streptococci, Listeriae, and Yersiniae. The DNA of the invention can be stably transfected into such a microbial strain by standard methods (Sambrook et al., Molecular Cloning: A Laboratory Manual. Cold Spring Harbor Lab. Press, New York), and then would be introduced into a mammal by, for example, oral or parenteral administration. Once introduced into the patient, the bacterium would multiply and express the mutant form of diphtheria toxin within the mammal, causing the mammal to maintain a protective level of antibodies to the mutant toxin. In a similar manner, an attenuated animal virus such as adenovirus, herpes virus, vaccinia virus, polio, fowl pox, or even attenuated eukaryotic parasites such as Leishmania can be employed as the carrier organism. The mutant DNA of the invention can be incorporated by genetic engineering techniques into the genome of any appropriate virus, which is then introduced into a vaccinee by standard methods. A live vaccine of the invention can be administered at, for example, about 10^(4–10) ⁸ organisms/dose, or a dose that is sufficient to stably induce protective levels of antitoxin. Optimal dosages of such a vaccine can be readily determined by one of ordinary skill in the field of vaccine technology.

OTHER EMBODIMENTS

Other embodiments are within the claims set forth below.

TABLE 1 Toxin Toxoid Intra- ID Anti- Anti- Vero Vero dermal Toxicity Toxin Toxoid Diphtheria Diphtheria Diphtheria MCD Fold- Toxicity Fold- Neut. Ab Neut. Ab IgG IgG Construct (μg) Reduction (μg) Reduction Titer Titer (μg/ml) (μg/ml) Native 0.000005 — 0.00005 — 0.18 0.14 — 550 ΨWT 0.009   2 × 10³ 0.02   8 × 10² 0.14 0.40 714 1277  Delta 148   2 4 × 10⁵ 50 1 × 10⁵ 0.01 ND 156 ND 516A/530A  140 3 × 10⁷  5 1 × 10⁴ 0.02 0.2  239 991 349K 1000 2 × 10⁸ 10 2 × 10⁴ 0.06 0.2  832 778 ΨWT 0.018   4 × 10³ ND ND 148K/51E 2800 6 × 10⁸ 50 >1 × 10⁵  190S/192G/193S/349K 1000 2 × 10⁸ 50 >1 × 10⁵  148S/516A/530A/349K 1300 3 × 10⁸ 50 >1 × 10⁵  

1. DNA encoding a polypeptide comprising a mutant diphtheria toxin C domain, a mutant diphtheria toxin T domain, and a mutant diphtheria toxin R domain, said C domain comprising a mutation in Glu148, said T domain comprising a mutation in Glu349, and said R domain comprising a mutation in one or both of Lys516 and Phe530 of wild-type diphtheria toxin.
 2. The DNA of claim 1, wherein said mutant C domain comprises the mutation Glu148Ser.
 3. The DNA of claim 1, wherein said mutant T domain comprises the mutation Glu349Lys.
 4. The DNA of claim 1, wherein said mutant R domain comprises the mutation Lys516A1a or the mutation Phe530A1a or both.
 5. The DNA of claim 1, wherein said DNA further encodes a second polypeptide.
 6. The DNA of claim 1, wherein said C domain comprises the mutation G1u148Ser, T comain comprises the mutation G1u349Lys, and R domain comprises the mutations Lys516Ala and Phe530Ala.
 7. A cell comprising the DNA of claim
 1. 8. The cell of claim 7, wherein said cell is a B. subtilis, BCG, Salmonella sp., Vibrio cholerae, Listeriae, Yersiniae, Streptococci, Corynebacterium diphtheriae, or an E. coli cell. 